Title:Streams and caustics: the fine-grained structure of LCDM haloes

Abstract: We present the first and so far the only simulations to follow the
fine-grained phase-space structure of galaxy haloes formed from generic LCDM
initial conditions. We integrate the geodesic deviation equation in tandem with
the N-body equations of motion, demonstrating that this can produce numerically
converged results for the properties of fine-grained phase-space streams and
their associated caustics, even in the inner regions of haloes. Our effective
resolution for such structures is many orders of magnitude better than achieved
by conventional techniques on even the largest simulations. We apply these
methods to the six Milky Way-mass haloes of the Aquarius Project. At 8 kpc from
halo centre a typical point intersects about 10^14 streams with a very broad
range of individual densities; the ~10^6 most massive streams contribute about
half of the local dark matter density. As a result, the velocity distribution
of dark matter particles should be very smooth with the most massive
fine-grained stream contributing about 0.1% of the total signal. Dark matter
particles at this radius have typically passed 200 caustics since the Big Bang.
The peak densities on present-day caustics in the inner halo almost all lie
well below the mean local dark matter density. As a result caustics provide a
negligible boost (<0.1%) to the predicted local dark matter annihilation rate.
The effective boost is larger in the outer halo but never exceeds about 10%.
Thus fine-grained streams and their associated caustics have no effect on the
detectability of dark matter, either directly in Earth-bound laboratories, or
indirectly through annihilation radiation, with the exception that resonant
cavity experiments searching for axions may see the most massive local
fine-grained streams because of their extreme localisation in energy/momentum
space. (abridged)